Biological membranes have excellent water transport characteristics, with certain membranes able to control permeability over a wide range. Membranes such as those present in the proximal tubules of the human kidney can be induced to insert specific water channel membrane proteins known as Aquaporins (AQPs) to increase permeability. (Knepper, M. A.; et al., “Renal Aquaporins,” Kidney Int 1996, 49, (6), 1712-7). Other biological membranes, such as those in mammalian optic lenses, erythrocytes, and other cell membranes, are constitutively AQP rich. (Gorin, M. B.; et al., “The major intrinsic protein (MIP) of the bovine lens fiber membrane: Characterization and structure based on cDNA cloning,” Cell 1984, 39, (1), 49-59). Permeabilities observed in AQP-rich membranes are orders of magnitude higher than those observed for unmodified phospholipid membranes (Borgnia, M. J.; et al., “Functional reconstitution and characterization of AqpZ, the E-coli water channel protein,” Journal of Molecular Biology 1999, 291, (5), 1169-1179).
Additionally, some members of the AQP family have excellent solute retention capabilities for very small solutes such as urea, glycerol and glucose even at high water transport rates (Borgnia, et al. (1999); Meinild, A. K.; et al., “Bidirectional water fluxes and specificity for small hydrophilic molecules in aquaporins 0-5,” Journal of Biological Chemistry 1998, 273, (49), 32446-32451). These properties result from the unique structure of the water-selective AQPs. AQPs have six membrane-spanning domains and a unique hourglass structure (Jung, J. S.; et al., “Molecular structure of the water channel through aquaporin CHIP. The hourglass model,” J Biol Chem 1994, 269, (20), 14648-54) with conserved charged residues that form a pore that allows the selective transport of water while rejecting solutes.
The effects of AQPs on the permeability of biological and synthetic lipid membranes has been studied by incorporating these proteins into liposomes (Borgnia, et al. (1999)), frog oocytes (Preston, G. M.; et al., “Appearance of Water Channels in Xenopus Oocytes Expressing Red-Cell Chip28 Protein,” Science 1992, 256, (5055), 385-387) and cellular secretory vesicles (Coury, L. A.; et al., “Use of yeast secretory (sec) vesicles to express and characterize aquaporin (AQP) 1 and 2 water channels,” Journal of the American Society of Nephrology 1996, 7, (9), A0088-A0088). However, the direct use of biological membranes or synthesized lipid membranes for water treatment and drug delivery applications has practical disadvantages. The major limitation is the low stability of lipid membranes. (Duncan, R., “The dawning era of polymer therapeutics,” Nat Rev Drug Discov 2003, 2, (5), 347-60). Obtaining and processing large volumes of such membranes would also present technical challenges.